1. Field of the Invention
The invention relates to a pyrogenic silica, the process for the production thereof and uses thereof.
2. Description of Related Art
All highly disperse silicas obtained through the coagulation of monomeric silicas in the gaseous phase at elevated temperature are classed under the term pyrogenic silica (also fumed silica). There are two processes for the industrial production of the pyrogenic silicas, namely high-temperature hydrolysis and the arc process.
In the process involving high-temperature hydrolysis, a homogeneous mixture consisting of vaporous silicon tetrachloride, hydrogen, oxygen and an inert gas undergoes combustion by a burner in a cooled combustion space. In the course of this the following reactions take place simultaneously: EQU 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl
Owing to the homogeneity of the gas mixture, the reaction conditions and therefore the conditions for formation and growth are largely the same for each particle of SiO.sub.2, so that very uniform and even particles can form (Winnacker-Kuchler "Chemische Technologie", Volume 2, Anorganische Technologie II, 4th Edition, page 77 (1983)). In the known process air is used as the oxygen source. The pyrogenic silicas produced by the known process have specific surfaces of between 90 and 600 m.sup.2 /g.
The production of pyrogenic silicas having a lower surface area may require an alteration of the procedure in such a way that the oxygen content of the air used for the reaction is definitely increased by adding further oxygen. By this means the proportion of ballast gas, for example the nitrogen from the air, in the combustion gases is lowered.
This process has the disadvantage that additional gas supply equipment, which is subject to special safety regulations, must be installed for the supply of additional oxygen.
This process moreover has the disadvantage that silicas of low surface area and having in addition a lowered DBP number cannot be produced.